Basically, if the primary and secondary coils are in phase with each other so that H1 and X1 are on the same "side" of the transformer, then the sine wave produced on the secondary winding will be in phase with the sine wave applied to the primary winding. However, if the secondary coil is "flipped" such that H1 and X2 are on the same "side" of the transformer, then the sine wave produced on the secondary will be 180 degrees out of phase with the sine wave applied to the primary. The same result can be achieved by simply swapping the connections on one side or the other of the transformer. (That is, swapping H1 and H2 OR swapping X1 and X2).